Arya, or more profoundly known as A, was the prodigy, the genius. All of the children that resided at Whammy's house were well aware of their job as Arya's successor. It was considered an honor to be a successor for one of the 'Lettered Kids.' There was usually only one LK per generation, and that's why it's odd when a boy named L shows up, and shakes things up for A.